A single optical signal channel can be transmitted through a cascade of quite a number of optical amplifiers before it requires full regeneration. The spectral gain characteristic of a typical optical amplifier, such as an erbium doped fibre amplifier, is wide enough to support amplification of several wavelength multiplexed signal channels. However the spectral gain characteristic is not entirely uniform over its full usable spectral range. Thus an erbium doped fibre amplifier may show a significant peak in its gain characteristic near the short wavelength end of that characteristic. The use of filters to flatten such characteristics is known, but is still liable to leave the characteristic with residual undulations. Therefore, if a wavelength multiplexed set of signal channels is transmitted through a concatenation of such amplifiers, those channels registering with any slight troughs in the gain characteristic will not be amplified as much as other channels registering with any slight peaks. Quite a small difference in gain per amplifier can become quite significant in a long concatenation of amplifiers because the difference is multiplied by the number of amplifiers. Thus for instance if there is 0.5 dB difference in gain per amplifier between a channel at wavelength .lambda..sub.1 and a channel at wavelength .lambda..sub.2 then, if both channels were launched with equal power, after passage through a concatenation of 20 such amplifiers, those channels would differ in power by 10 dB, thereby bringing the weaker channel much closer to, or even beneath, the noise floor.